Journal Article written by Patrice Umenne in the department of Electrical and Mining EngineeringIn this paper we report on the development of a new application of the femtosecond laser, namely, the
fabrication of micron, sub-micron and nano sized constrictions on YBCO superconductive thin films. The fabricated
micron, sub-micron and nano constrictions have widths of 1.09 um, 705 nm and 394.8 nm, respectively. The
thickness of these constrictions ranged from about 160 nm to 190 nm, as defined by the depth profile on the 3D
AFM images. The length of these constrictions ranged from 7.2 um to 7.5 um. The length of these constrictions
depends on the size of the laser ablation spot used for cutting and on the separation distance between the laser
ablation spots along the width of the sample.The University of South AfricaCollege of Engineering, Science and Technolog

Patrice Umenne

V.V Srinivasu

Acta Physica Polonica A

English

Unisa Institutional Repository

Vol. 134 (2018) ACTA PHYSICA POLONICA A No. 1Special Issue of the 7th International Advances in Applied Physics and Materials Science (APMAS 2017)Constrictions Fabricated on YBCO Thin Filmusing the Femtosecond Laser: Limiting FactorsP.O. Umennea and V.V. Srinivasub,∗aDepartment of Electrical and Mining Engineering, University of South Africa, Johannesburg 1710, South AfricabDepartment of Physics, University of South Africa, Johannesburg 1710, South AfricaIn this paper we report on the development of a new application of the femtosecond laser, namely, thefabrication of micron, sub-micron and nano sized constrictions on YBCO superconductive thin films. The fabricatedmicron, sub-micron and nano constrictions have widths of 1.09 µm, 705 nm and 394.8 nm, respectively. Thethickness of these constrictions ranged from about 160 nm to 190 nm, as defined by the depth profile on the 3DAFM images. The length of these constrictions ranged from 7.2 µm to 7.5 µm. The length of these constrictionsdepends on the size of the laser ablation spot used for cutting and on the separation distance between the laserablation spots along the width of the sample.DOI: 10.12693/APhysPolA.134.100PACS/topics: femtosecond laser, nanostructures, atomic force microscope, Josephson junctions1. IntroductionThe femtosecond laser can be used in many appli-cations such as micromachining of bulk glass [1], writ-ing waveguides in glass [2], polymerization of inorganic-organic hybrid materials [3], as a high accuracy opticaldistance measuring tool [4], in the medical field for eyecorneal refractive surgery [5], for the fabrication of tem-perature sensors [6], for colorizing metals [7], etc.In this paper we report on a new application. Weuse the ablative properties of the femtosecond laser tofabricate constrictions on a superconductive YBCO thinfilm. These constrictions could later be used as Joseph-son junctions. Josephson junctions have many appli-cations of their own, such as SQUID’s magnetic sen-sors and as elements of qubit technology. In addi-tion, the limiting factors for the fabrication of “nano”constrictions are discussed.2. Experimental details2.1. Laser specifications and settingsThe employed femtosecond laser has a wavelength of795 nm and a pulse duration time of 130 fs. The laserablation width achieved using a spherical convex lens of30 mm focal length was on average 15.53 µm. Whenthe separation distance between the laser ablation spotsSW was set to 16.5 µm the sub-micron sized constrictionof 705 nm was produced. With a separation distance be-tween the laser ablation spots SW of 16.0 µm we achievedthe nano sized constriction of 394.8 nm. Lastly with aseparation distance of 17.0 µm we achieve a micron sized∗corresponding author; e-mail: vallavs@unisa.ac.zaconstriction with a width of 1.09 µm. This dependenceis summarized in Eq. (1);constrict. width = SW − laser ablation spot size. (1)The fabricated constrictions where S-shaped. With suchshape it is possible to regulate both the width and thelength of the constriction, as described elsewhere [8]. Thewidth is controlled by bringing the laser ablation spotscloser along the length of the thin film sample, while thelength can be controlled by separating the laser ablationspots along the width of the thin film sample.However, as the laser ablation spots are brought closertogether along the length of the sample the constrictionwidth tends to become smaller until in some instancesthe fabricated constrictions would collapse.3. Results and discussion3.1. Factors that limit the fabrication of nanostructuresWhen nano-structuring a material using the femtosec-ond laser, the width of the constriction or the diameterof the smallest structure that can be fabricated is deter-mined by Eq. (2) below;d = kλ/NAq1/2, (2)where d is the diameter of the constriction made or thewidth, λ is the wavelength of the femtosecond laser, NA isthe numerical aperture of an objective plano convex lensof the focusing optics, q is the energy band gap of thematerial sample being used for nano structuring. Thisequation was obtained from [9]. The main limiting fac-tors of nano-fabrication are discussed below.1. One of the main limiting factors for the fabricationof nano bridges is in the specification of the lens beingused for optimizing the size of the laser ablation spotduring focusing of the laser beam. The laser ablationspot size has to be as small as possible before cutting theconstriction on the sample. The conventional spherical(100)Constrictions Fabricated on YBCO Thin Film using the Femtosecond Laser. . . 101convex lenses, such as those with a relative focal length,place a limit on the size of the laser ablation spot they canproduce because of their reduced focusing power. Thesmaller the laser ablation spot size, the smaller the con-striction that can be fabricated. This is because if a con-striction of nano range has to be made, then the spot sizehas to be in the 1–5 µm range, so that the laser ablationspots can be brought closer to one another without clip-ping the constriction bridge. The objective plano convexlenses with such numerical apertures NA as 0.25, 0.40,0.65 are more suitable to produce smaller laser ablationspot diameters and hence smaller constrictions becausethey are able to converge the laser beam more. Hencethey are able to produce smaller ablations spots.2. The energy band gap of the material being usedfor nano-structuring, in this case the YBCO thin film,also plays a role. The materials with a higher energyband gap tend to have the capability of producing smallerconstriction sizes. For example YBCO thin film has anout-of-plane energy band gap of 5 meV [10] which allowsit to produce nano structures somewhere in the limit of400 nm. This could be one of the reasons why when theconstriction of nano size (394.8 nm) was produced, thewalls of the constriction had a tendency to collapse afterfabrication. Other factors that contribute towards thecollapse of the constriction include how close the abla-tion spots are brought to each other, vibration and thetemperature in the clean room while cutting.3. The last limiting factor is the radiation wavelengthof the laser being used. The femtosecond laser used forthis experiment had a radiation wavelength λ of 795 nm.Essentially this places a diffraction limit on the light thatis incident on the sample from the laser when cutting.This limit also reduces the chances of cutting constric-tions with diameter less than 795 nm without collapsingthe constriction.3.2. Nano constriction (0–500 nm)The nano sized constriction was fabricated with thefollowing laser settings; separation distance between thelaser ablation spots SW = 16.0 µm, the laser ablationspot size of 15.53 µm, laser power setting 2.035 mWand in the optical set up a spherical convex lens of fo-cal length of 30 mm was utilised. The constriction widthwas measured and was determined to be 394.8 nm usingthe amplitude line fit, as can be seen in Fig. 1b. Thescan area in Fig. 1 was set to 20.2 × 20.2 µm2 usingthe AFM software [11], which enables one to see thatthe constriction is clipped and not geometrically contin-uous. Figure 1a shows the topography line fit and Fig. 1bshows the amplitude line fit used to determine the widthof the constriction.3.3. Sub-micron sized constriction (500–1000 nm)For the sub-micron sized constriction the separationdistance between laser ablation spots SW was 16.5 µm.The laser ablation spot size was 15.82 µm. The fabricatedconstriction is geometrically continuous as can be seen inFig. 1. AFM image of the nano constriction with thewidth of 394.8 nm.Fig. 2a. The scan area was set to 16.4 × 16.4 µm2 andthe constriction width was measured to be 705 nm usingthe amplitude line fit of the constriction in Fig. 2b.Fig. 2. AFM image of the sub-micron constriction withthe width of 705 nm (shown by the black arrows in theprofile image).3.4. Micron sized constriction (above 1000 nm)The micron sized constriction was fabricated withthe separation distance between the laser ablation spotsSW = 17.0 µm. The laser ablation spot size was15.23 µm, which is slightly less than the ablation spotsize for the nano constriction. The laser power was set to2.0 mW and a spherical convex lens with a focal length102 P.O. Umenne, V.V. Srinivasuof 30 mm was used. The constriction width was deter-mined to be 1.09 µm, as can be seen from the amplitudeline fit in Fig. 3b. The fabricated constrictions and theirphysical dimensions are summarized in Table I. Figure 3ashows the topography line fit of the constriction.Fig. 3. AFM image of the micron constriction with thewidth of 1.09 µm.TABLE ISummary of prepared constrictions with allspecifications.Constriction type Nano Sub-micron MicronNumber 1 2 3SW [µm] 16.0 16.5 17.0Qubit number on YBCO films 3rd 5th 4thConstriction width [nm] 394.8 705 1090Ablation spot diameter [µm] 15.53 15.8 15.234. ConclusionsWe demonstrated a new application of femtosecondlaser in fabrication of micron, sub-micron and nano sizedconstrictions that could be used as Josephson junctions.A 395 nm, 705 nm and 1.09 µm wide constrictions wherefabricated. The limitations of using the femtosecond laseras a nano structuring tool where discussed. The separa-tion distances between the laser ablation spots SW andthe ablation spot sizes were determined experimentallyfor each constriction in order to be able to fabricate theseconstrictions.References[1] C.B. Schaffer, A. Brodeur, J.F. García, E. Mazur,Opt. Lett. 26, 93 (2001).[2] K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt.Lett. 21, 1729 (1996).[3] J. Serbin, A. Egbert, A. Ostendorf, B.N. Chichkov,R. Houbertz, G. Domann, J. Schulz, C. Cronauer,L. Fröhlich, M. Popall, Opt. Lett. 28, 301 (2003).[4] K. Minoshima, H. Matsumoto, Appl. Opt. 39, 5512(2000).[5] A. Sugar, Curr. Opin. Ophthalmol. 13, 246 (2002).[6] D. Grobnic, S.J. Mihailov, C.W. Smelser, H. Ding,IEEE Photonics Technol. Lett. 16, 2505 (2004).[7] A.Y. Vorobyev, C. Guo, Appl. Phys. Lett. 92, 41914(2008).[8] P. Umenne, V.V. Srinivasu, Journal of Materials Sci-ence: Materials in Electronics 28, 5817 (2008).[9] F. Korte, J. Serbin, J. Koch, A. Egbert, C. Fallnich,A. Ostendorf, B.N. Chichkov, Appl. Phys. A 77, 229(2003).[10] N. Hass, G. Deutscher, G. Desgardin, I. Monot,M. Weger, Journal of Superconductivity 5, 191 (1992).[11] Nanosurf FlexAFM, Operating Instructions Versi-on 3, NANOSURF 2011.

Constrictions Fabricated on YBCO Thin Film using the Femtosecond Laser: Limiting Factors.

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Constrictions Fabricated on YBCO Thin Film using the Femtosecond Laser: Limiting Factors.

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